GIS Glossary

Accuracy: The closeness or nearness of the measurements to the true or actual value of the quantity being measured. Usually represented as a percentile.

Accuracy, absolute: The accuracy of a map in representing the geographic location of an object relative to its true location on the surface of the Earth. Absolute accuracy is based on geographic coordinates.

Accuracy, relative: The accuracy of a map in representing the geographic location of an object relative to the locations of other objects.

American Standard Code for Information Interchange (ASCII): A code for representing English characters as numbers, with each character assigned a number from 0-127. ASCII file A text file in which each byte represents one character according to the ASCII code. ASCII files are sometimes called plain text files.

Annotation: Descriptive text used to label coverage features.

Arc: A string of x,y coordinate pairs (vertices) that begin at one location and end at another. Connecting the arc's vertices creates a line.

Arc Attribute Table (AAT): A table containing attributes for a line coverage such as streets or streams.

ARC Macro Language (AML): A high level language that provides full programming capabilities and a set of tools for building menus to tailor user interfaces for specific applications.

Attribute: A characteristic of a geographic feature described by numbers or letters, typically stored in tabular format and linked to the feature in a relational database. The attributes of a well represented by a point might include depth, location, and permit number.

Cadastral: Relating to land boundaries and subdivisions, parcels of land suitable for transfer of title. The legally recognized registration of the quantity, value and ownership of land parcels.

Cadastre: Public record of the extent, value and ownership of land within a district for purposes of taxation. The cadastre layer includes parcel limits, subdivision limits, political boundaries, and the land survey boundaries. This layer is continually updated.

Cartesian Coordinate System: A two dimensional coordinate system in which x measures horizontal distance and y measures vertical distance. An x,y coordinate defines every point on the plane.

Clip: The spatial extraction of those features from one coverage that reside entirely within the boundary defined by features in another coverage. Clipping works much like a cookie cutter.

Computer Aided Design (CAD): An automated system for the design, drafting and display of graphically oriented information.

Contour Line: An imaginary line joining points of equal elevation.

Control Points: A set of points on the ground whose horizontal and vertical location is known. Control points are used as the basis for detailed surveys.

Coordinate: An x,y location in a Cartesian coordinate system or an x,y,z coordinate in a three dimensional system. Coordinates represent locations on the Earth's surface relative to other locations.

Coordinate Geometry (COGO): A method of defining geometric features through the input of bearing and distance measurements. Coordinate Geometry (COGO) functions are typically used by land surveyors to enter traverses around spatial features such as parcels, to calculate precise locations and boundaries sing distances and bearings from reference points, and to define curves using a point location, radius, arc-length, and so on.

Coordinate System: The system used to measure horizontal and vertical distances on a planimetric map. A common coordinate system is used to spatially register geographic data for the same area.

Coverage: A digital version of a map forming the basic unit of vector data storage in ARC/INFO. A coverage stores map features as primary features (such as arcs, nodes, polygons, and label points) and secondary features (such as tics, map extent, links, and annotation). Associated feature attribute tables describe and store attributes of the map features. A coverage usually represents a single theme, or layer, such as soils, roads, or land use.

Coverage Units: The units (e.g., feet, meter, inches) of the coordinate system in which a coverage is stored.

Data conversion: The translation of data from one format to another. ARC/INFO supports data conversion from many different geographic data formats in addition to routines for converting paper maps. Those data formats include DLG, TIGER, DXF, and DEM.

Data Integrity: Refers to the consistency and accuracy of data that is stored in a database.

Database: A logical collection of interrelated information, managed and stored as a unit. A GIS database includes data about the spatial location and shape of geographic features recorded as points, lines, and polygons as well as their attributes.

Database Management System (DBMS): System providing for the input, storage and retrieval of data.

Datum: A set of parameters and control points used to accurately define the three dimensional shape of the Earth. The corresponding datum is the basis for a planar coordinate system. For example the North American Datum, NAD83 is the horizontal datum and North American Datum NAVD88 is the vertical datum for coordinates used in Delaware County's GIS.

Differential Global Positioning System (DGPS): A positioning procedure that uses two receivers, a rover at an unknown location and a base station at a known, fixed location. The base station computes corrections based on the differences between its actual and observed ranges to the satellites being tracked.

Digital Elevation Model (DEM): A topographic surface arranged in a data file as a set of regularly spaced x, y, z coordinates where z represents elevation.

Digital Line Graph (DLG): The digital format standards published by USGS for exchange of cartographic data files and for which Digital Line Graph data sets are delivered by USGS. Digital Line Graph files from the U.S. Geological Survey. These files include digital information from the USGS map base categories, such as transportation, hydrography, contours and public land survey boundaries.

Digital Map Library: A series of directories and subdirectories designed to uniformly organize a collection of spatial data. Map libraries organize geographic data spatially as a set of tiles and thematically as a set of layers. Volusia County's digital map library is divided into large scale and small scale subdirectories containing several hundred tiles comprising 90 layers of information.

Digitize: To encode map features as x,y coordinates in digital form. Lines are traced to define their shapes. This can be accomplished either manually or by use of a scanner. To encode map features as x,y coordinates in digital form. Lines are traced to define their shapes. This can be accomplished either manually or by use of a scanner.

Dissolve: The process of removing boundaries between adjacent polygons that have the same values for a specified attribute. The process of removing boundaries between adjacent polygons that have the same values for a specified attribute.

Drawing Exchange Format (DXF): An exchange format for CAD files. A format for storing vector data in ASCII or binary files; used by AutoCAD and other CAD software and convertible to ARC/INFO coverages.

Dynamic Segmentation is a two-step process performed on a spatial data set comprised of linear features. First, a route system is created by associating adjacent line segments into one or more groups that have a definite linear sequence. Second, descriptive information is associated with the route system by referencing distances from the starting point of each route. For example, a stream route system is created by grouping stream segments into routes that represent the mainstem, tributaries, and headwater reaches. Spawning habitat areas are then mapped by their locations along the routes. The advantage of using dynamic segmentation is that small areas along a line feature can be referenced without actually breaking the line into pieces. Also, linear distances, such as river miles, can then be calculated directly from the routes and associated attributes.

Generalization: Removal of detail from a data layer to make processing or visualization easier at smaller scales.

Geocode: The process of identifying a location by one or more x,y coordinates from another location description such as an address. For example, an address can be matched against Volusia County's street centerline file to determine an x,y coordinate.

Georeference: To establish the relationship between page coordinates on a planar map and real-world coordinates.

Geo-stationary Satellites that maintain a stationary position relative to the earth.

Global Positioning System (GPS): A satellite based device that records x,y,z coordinates and other data. GPS devices can be taken into the field to record data while driving, hiking, or flying. Ground locations are calculated by signals from satellites orbiting the Earth.

Land Information Systems (LIS): Specialized GIS systems most commonly found among municipal agencies geared toward legal, administrative and economic spatial analysis. Also known as Land Records Information System (LRIS).

Land Records Information System (LRIS): See Land Information Systems.

Lot Government: lot or subdivision lot representing the boundary of a legally conveyable unit of land identified on a record document. A lot may or may not be coterminous with an Assessor parcel.

Latitude-Longitude: A spherical reference system used to measure locations on surface. Latitude measures angles in the north south direction and longitude measures angles in the east west direction. A spherical reference system used to measure locations on surface. Latitude measures angles in the north south direction and longitude measures angles in the east west direction.

Layer: A logical set of thematic data described and stored in a map library. Layers organize a map library by subject matter, e.g., soils, roads, wells, and extend over the entire geographic area defined by the spatial index of the map library.

Line-In-Polygon: A spatial operation in which arcs in one coverage are overlaid with polygons in another to determine which arcs, or portions of arcs, are contained within the polygons. Polygon attributes are associated with corresponding arcs in the resulting line coverage.

Logical Selection: The process of selecting a subset of features from a coverage using logical selection criteria that operate on the attributes of coverage features (e.g., area greater than 16,000 square feet). Only those features whose attributes meet the selection criteria are selected. Also known as feature selection by attribute.

Many-To-One-Relate: A relate in which many records in one table are related to a single record in another table. A goal in relational database design is to use one to many relates to reduce data storage and redundancy. A relate in which many records in one table are related to a single record in another table. A goal in relational database design is to use one to many relates to reduce data storage and redundancy.

Map Extent: The rectangular limits (xmin,ymin,xmax,ymax) of the area of the Earth's surface you want to display using ARC/INFO. The geographic extent specified by the minimum bounding rectangle of a study area. extent The rectangular limits (xmin,ymin,xmax,ymax) of the area of the Earth's surface you want to display using ARC/INFO. The geographic extent specified by the minimum bounding rectangle of a study area.

Map Gallery: A collections of maps sometimes grouped by a common theme. Map galleries are a popular method of showcasing. You'll often find them at Geography related conferences. Ever increasing are online map galleries.

Map Projection: A systematic conversion of locations on the Earth's surface from spherical to planar coordinates. Several of the more popular projections are: State Plane Coordinates (SPC) which uses feet for units of measure; Universal Transverse Mercator (UTM) which uses meters for units of measure; and latitude and longitude which uses degrees, minutes, and seconds of arc for units of measure.

Map Scale: The extent of reduction needed to display a representation of the Earth's surface on a map. A statement of a measure on the map and the equivalent measure on the Earth's surface, often expressed as a representative fraction of distance, such as 1:24,000 (one unit in the map equals 24,000 units on the ground). The extent of reduction needed to display a representation of the Earth's surface on a map. A statement of a measure on the map and the equivalent measure on the Earth's surface, often expressed as a representative fraction of distance, such as 1:24,000 (one unit in the map equals 24,000 units on the ground).

One-To-Many-Relate: A type of relate connecting a unique value in one file to many records (that have the same value) in another file.

Orthophotography: The process of aerial photographs that have been rectified to produce an accurate image of the Earth by removing tilt and relief displacements which occurred when the photo was taken. The digital orthophotography were taken during three separate years. The southern third of the county were taken in 1993. The central third of the county were taken in 1994. The northern third of the county were taken in 1995.

Point-In-Polygon: A spatial operation in which points from one coverage are overlaid with a polygonal coverage to determine which points fall within the polygon boundaries. Points assume the attributes of the polygons within which they fall.

Political Boundaries include those lines depicting the boundaries of the County, townships, and municipalities.

Polygon: A multisided figure that represents area on a map. A feature defined by the arcs that make up its boundary. Every polygon contains one label point within its boundary. Polygons have attributes that describe the geographic feature they represent.

Polygon Overlay: A process that merges spatially coincident polygons from two coverages, and their attributes, to create a third coverage, that contains new polygons and describes new relationships.

Precision: The closeness with which the measurements agree with each other.

Public Land Survey System (PLSS): A rectangular system of land surveys. The land was surveyed and marked by durable monuments into an orderly arrangement of squares, according to rigidly standardized directions from the government. Various starting points were established throughout the western land. Through each point, an east-west line, called a baseline, was marked on the ground, usually for many tens or hundreds of miles in both directions, and a north-south line, called a meridian or range, was similarly marked. From the intersection of the baseline and the meridian, or range, six-mile squares, called townships, were laid out and monumented. Each township is identified by a "township" or "T" number according to how far east or west it is of the meridian. Each township is divided into thirty-six one-mile squares, called sections, which are numbered. Each section may be divided into halves, quarters, and smaller subdivisions, all according to rigidly standardized directions from the government. Irregularly shaped subdivisions may be called lots. Around three-fourths of Delaware County is based on the Public Land Survey System (plss). The remaining one-fourth of the County is based on the Virginia Military System (VMS).

Relate: An operation that establishes a temporary connection between corresponding records in two tables using an item common to both. A relate gives access to additional feature attributes that are not stored in a single table. An operation that establishes a temporary connection between corresponding records in two tables using an item common to both. A relate gives access to additional feature attributes that are not stored in a single table.

Relate Key: The common set of columns used to relate two attribute tables.

Relational Database: A collection of data organized in two-dimensional tables consisting of named columns and (usually unique) rows.

Remotely Sensed Data: Data collected from a distance.

Remote Sensing: The technique of collecting information from a distance. Most common mediums include satellite imaging, Global Positioning Systems, RADAR, SONAR, aerial photography, etc. Before the advent of photography, remote sensing was accomplished in such ways as information collection from air balloons.

Resolution: Measures the sharpness of an image.

Right of Way (ROW): Land area providing legal right of passage, i.e., street right-of-way, railroad right-of-way, etc.

The relationship between the size of the map and the corresponding size of the real world.

A level of resolution or zoom.

Selective: Availability (S/A): A U.S. Department of Defense program to limit the accuracy of autonomous position fixes computed by civilian receivers. The error in position caused by S/A can be up to 100 meters.

Slope: A measure of change in surface value over distance, expressed in degrees or as a percentage. For example, a rise of 2 meters over distance of 100 meters describes a 2% slope.

Slope Rate of change in elevation.

Soil is a natural, three-dimensional body on the earth's surface that supports plants and that has properties resulting from the integrated effect of climate and living matte acting on earthy parent material, as conditioned by relief over periods of time.

Soil Series is a group of soils developed from a particular type of parent material and having genetic horizons that, except for texture of the surface layer, are similar in differentiating characteristics and in arrangement in the profile.

Soil Survey Geographic (SSURGO): Data Base national standards used to construct the soil maps in the Soil Survey Geographic (SSURGO) data base. Mapping scales generally range from 1:12,000 to 1:63,360; SSURGO is the most detailed level of soil mapping done by the Natural Resources Conservation Service (NRCS). SSURGO digitizing duplicates the original soil survey maps. This level of mapping is designed for use by landowners, townships, and county natural resource planning and management. The user should be knowledgeable of soils data and their characteristics. Digitizing is done by line segment (vector) format in accordance with Natural Resources Conservation Service (NRCS) digitizing standards. The mapping bases meet national map accuracy standards and are either orthophotoquads or 7.5-minute topographic quadrangles. SSURGO data are collected and archived in 7.5-minute quadrangle units, and distributed as complete coverage for a soil survey area. Soil boundaries ending at quad neatlines are joined by computer to adjoining maps to achieve an exact match. SSURGO is linked to a Map Unit Interpretations Record (MUIR) attribute data base. The attribute data base gives the proportionate extent of the component soils and their properties for each map unit. The SSURGO map units consist of 1 to 3 components each. The Map Unit Interpretations Record data base includes over 25 physical and chemical soil properties.

Soil Survey Manual provides the major principles and practices needed for making and using soil surveys and for assembling and using data related to them. The Manual is intended primarily for use by soil scientists engaged in the classification and mapping of soils and in the interpretation of soil surveys. Although the Manual is oriented to the needs of those actively engaged in preparing soil surveys for publication, workers and students who have limited soils experience or are less familiar with the soil survey process also will be able to use the information.

Spatial Analysis: The process of modeling, examining, and interpreting model results. Spatial analysis is the process of extracting or creating new information about a set of geographic features. Spatial analysis is useful for evaluating suitability and capability, for estimating and predicting, and for interpreting and understanding. In GIS there are four traditional types of spatial analysis: spatial overlay and contiguity analysis, surface analysis, linear analysis, and raster analysis. The process of modeling, examining, and interpreting model results. Spatial analysis is the process of extracting or creating new information about a set of geographic features. Spatial analysis is useful for evaluating suitability and capability, for estimating and predicting, and for interpreting and understanding. In GIS there are four traditional types of spatial analysis: spatial overlay and contiguity analysis, surface analysis, linear analysis, and raster analysis.

Spatial Data Transfer Standard (SDTS): A comprehensive transfer standard for Earth-referenced data which may be used to transfer all types of spatial data between dissimilar computer systems.

Spatial Modeling: Analytical procedures applied with GIS. There are three categories of spatial modeling functions that can be applied to geographic data within a GIS: geometric models, such as calculating the distance between features, generating buffers, calculating areas and perimeters, and so on; coincidence modeling, such as polygon overlay; and adjacency modeling such as redistricting and allocation.

Structured Query Language (SQL): A syntax for defining and manipulating data from a relational database. Developed by IBM in the 1970s, it has become an industry standard for query languages in most relational database management systems.

State Plane Coordinates (SPCS): The State Plane Coordinate System is not a projection; rather it is a system for specifying positions of geodetic stations using plane rectangular coordinates. This coordinate system divides all fifty states of the United States, Puerto Rico and the U.S. Virgin Islands into over 120 numbered sections, referred to as zones. By providing an SPC easting (x) and northing (y), the state name, and the zone number, any location in the United States can be identified by a unique coordinate value. SPC Zone boundaries follow state and county boundaries. Florida, due to its size and shape is divided into three SPC zones, north, east, and west. Volusia County is entirely within the East Zone. State Plane Coordinates are admirably suited to the needs of the local land surveyor and are widely used for public works, land surveys, and for Geographic Information Systems.

State Soil Geographic Database (STATSGO): State general soil maps made by generalizing the detailed soil survey data. The level of mapping is designed to be used for broad planning and management uses covering state, regional, and multi-state areas. STATSGO data are designed for use in a Geographic Information System (GIS). STATSGO data are available in the USGS Digital Line Graph (DLG-3) optional distribution format. NRCS soil map symbols are not normally carried within the DLG-3 file; however, these map symbols are made available as a unique ASCII file when NRCS soils data are distributed in the DLG-3 format. STATSGO data are also available in ArcInfo 7.0 coverage and GRASS 4.13 vector formats.

Sun-Synchronous Orbit: An orbit that always passes over the earth at the same local sun time.

Tolerance: A numerical value representing the acceptable error range a feature will have from its actual point found on earth.

Topography: Shape or configuration of the land surface; represented in map form by contour lines. Delaware County's topographic layer consists of contour lines at two, five, and ten foot intervals.

Topology: The spatial relationships between connecting or adjacent coverage features.

Transformation: The process that converts coordinates from one coordinate system to another through translation, rotation, and scaling.

Triangulated Irregular Network (TIN): A series of triangles constructed using elevation data points taken from coverages. These triangles are used for surface representation and display.

Triangulation: A method of surveying in the location of an object may be calculated from the known locations of two other objects. Creating a triangle from the three items, the angles and sides of the triangle can be measured and the location of the unknown object is calculated algebraically.

Universal Transverse Mercator (UTM): The Universal Transverse Mercator is an international plane (rectangular) coordinate system developed by the U.S. Army. The UTM divides the world into 60 zones of 6 degrees longitude. Each zone extends 3 degrees east and west from its central meridian and are numbered consecutively west to east from the 180-degree meridian. Transverse Mercator projections may then be applied to each zone. (UTM) The Universal Transverse Mercator is an international plane (rectangular) coordinate system developed by the U.S. Army. The UTM divides the world into 60 zones of 6 degrees longitude. Each zone extends 3 degrees east and west from its central meridian and are numbered consecutively west to east from the 180-degree meridian. Transverse Mercator projections may then be applied to each zone.

Watershed: is an area of land where the majority of the water drains into the nearby streams, rivers or creeks. Similar to a large catch basin. Typically, a watershed boundary or perimeter is comprised of ridgetop or high elevation areas. All water runs downhill by the force of gravity, so you can think of a watershed as a drainage basin. Watersheds can be very large and can cover several states like the Columbia River watershed in Washington and Oregon states. Watersheds can also be very small, encompassing only a small stream or wetland area. Think of a watershed as the geographic area or boundary where the water drains to a specific river, lake, stream or any body of water.